The present invention generally relates to capstan motor control circuits, and more particularly to a capstan motor control circuit which controls a capstan motor depending on various modes of a video tape recorder (hereinafter simply referred to as a VTR).
A VTR has various reproducing modes such as a high-speed search mode, a normal reproduction mode, a low-speed search mode and a still reproduction mode. For example, the high-speed search mode includes modes such as a 20-times speed mode, a 15-times speed mode, a 5-times speed mode and a 2-times speed mode. On the other hand, the low-speed search mode includes modes such as a 1/3-speed search mode, a 1/10-speed search mode and a 1/30-speed search mode. The reproducing mode is selected by manipulating a search dial so that a rotational speed of a capstan motor and hence a transporting speed of a magnetic tape are variably set depending on the selected reproducing mode.
As will be described later on in the present specification in conjunction with a drawing, an example of a conventional capstan motor control circuit has a search dial which is manipulated to as to select a desired reproducing mode. When the search dial is manipulated to change the reproducing mode from a high-speed search mode to a still reproduction mode, a control device supplies to a capstan motor an instruction for rotating in a direction opposite to a direction in which the capstan motor has been rotating in response to an output of the search dial. On the other hand, a relatively large instruction voltage, that is, a braking voltage, is obtained from the control device and is supplied to the capstan motor via a differential amplifier and a driving amplifier. As a result, the rotational speed of the capstan motor is quickly decreased.
A frequency detecting element (frequency generator) detects the rotational speed of the capstan motor and generates a signal having a frequency dependent on the detected rotational speed. The output signal of the frequency detecting element is supplied to the control device and to a frequency-to-voltage converter which will be described later. Hence, when the rotational speed of the capstan motor decreases, the output signal frequency of the frequency detecting element decreases, and the control device detects the decrease in the output signal frequency of the frequency detecting element and supplies a zero instruction voltage to the capstan motor so as to stop the capstan motor.
Next, when the reproducing mode is changed from a high-speed search mode to a low-speed search mode, an instruction voltage having a level dependent on the speed of the selected low-speed search mode is obtained from the control device and is supplied to a non-inverting input terminal of the differential amplifier. In this case, the control device does not supply to the capstan motor an instruction for rotating in a direction opposite to a direction in which the capstan motor has been rotating. An output voltage of the frequency-to-voltage converter is supplied to an inverting input terminal of the differential amplifier. In this state, the voltage at the inverting input terminal of the differential amplifier is large compared to the voltage at the non-inverting input terminal thereof, and the output of the differential amplifier becomes zero. As a result, the supply of the driving voltage to the capstan motor is cut off, and the rotational speed of the capstan motor decreases gradually.
As the rotational speed of the capstan motor decreases, the output voltage of the frequency-to-voltage converter decreases gradually. When the output voltage of the frequency-to-voltage converter becomes approximately equal to the instruction voltage, the output voltage of the differential amplifier stabilizes to this instruction voltage and the capstan motor is driven by this instruction voltage.
Especially in the case where the reproducing mode is changed from the high-speed search mode to the low-speed search mode in the conventional capstan motor control circuit, the output of the differential amplifier is zero while the output voltage of the frequency-to-voltage converter is large compared to the instruction voltage. Since there are no braking voltage and no instruction for rotating the capstan motor in the opposite direction as in the case where the reproducing mode is changed from the high-speed search mode to the still reproduction mode, the conventional capstan motor control circuit suffers a problem in that it takes a long time for the VTR to assume the low-speed search mode from a time when the search dial is manipulated to change the reproducing mode.